The Electronic Behaviors of Oxygen-Deficient VO2 Thin Films in Low Temperature Region

The electronic properties of vanadium dioxide (VO2) thin films with various degrees of oxygen deficiency in the low temperature region were investigated by measurements of de and ac resistance R(omega). Sample films were deposited on sapphire and SiO2/Si substrates by laser ablation at 400 degrees C and 500 degrees C. R(omega)(-1) were fitted to the continuous time random walk ac conductivity to obtain the minimum hopping rate Gamma(min) for each deposition condition. By the comparison of Gamma(min) and its dependence on temperature T with de conductivity, together with the analysis of its weak T-m dependence, the major role of hopping in de conduction was revealed for samples at 400 degrees C, and disorder effects introduced by low deposition temperature were suggested in the oxygen-deficient VO2 films. Deviations of m value ranged from 1/4 in Mott's variable range hopping theory were found for samples showing the broadening of conductivity transition profile reported previously, and were discussed by considering disorder effects.